1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding polypeptides that are associated with root development, which contribute to plant growth and, ultimately affect plant production (i.e. yield) under abiotic stress or non-stress conditions. In particular, this invention relates to isolated nucleic acid sequences encoding polypeptides that confer upon the plant increased root growth, increased yield, and/or increased drought, cold, and/or salt tolerance, and the use of such isolated nucleic acids.
2. Background Art
The yield of crop plants is central to the well being of humans and is directly affected by the growth of plants under physical environment. Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as soybean, rice, maize (corn), cotton, and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plant biomass is the total yield for forage crops like alfalfa, silage corn and hay. Many proxies for yield have been used in grain crops. Chief amongst these are estimates of plant size. Plant size can be measured in many ways depending on species and developmental stage, but include total plant dry weight, above-ground dry weight, above-ground fresh weight, leaf area, stem volume, plant height, rosette diameter, leaf length, root length, root mass, tiller number and leaf number. Many species maintain a conservative ratio between the size of different parts of the plant at a given developmental stage. These allometric relationships are used to extrapolate from one of these measures of size to another. Plant size at an early developmental stage will typically correlate with plant size later in development. A larger plant with a greater leaf area can typically absorb more light and carbon dioxide than a smaller plant and therefore will likely gain a greater weight during the same period. This is in addition to the potential continuation of the micro-environmental or genetic advantage that the plant had to achieve the larger size initially. There is a strong genetic component to plant size and growth rate, and so for a range of diverse genotypes plant size under one environmental condition is likely to correlate with size under another. In this way a standard environment is used as a proxy for the diverse and dynamic environments encountered at different locations and times by crops in the field.
Harvest index, the ratio of seed yield to above-ground dry weight, is relatively stable under many environmental conditions and so a robust correlation between plant size and grain yield can often be obtained. These processes are intrinsically linked because the majority of grain biomass is dependent on current or stored photosynthetic productivity by the leaves and stem of the plant. Therefore, selecting for plant size, even at early stages of development, has been used as an indicator for future potential. When testing for the impact of genetic differences on stress tolerance, the ability to standardize soil properties, temperature, water and nutrient availability and light intensity is an intrinsic advantage of greenhouse or plant growth chamber environments compared to the field. However, artificial limitations on yield due to poor pollination due to the absence of wind or insects, or insufficient space for mature root or canopy growth, can restrict the use of these controlled environments for testing yield differences. Therefore, measurements of plant size in early development, under standardized conditions in a growth chamber or greenhouse, are standard practices to provide indication of potential genetic yield advantages.
During the life cycle, plants are typically exposed to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on development, growth, plant size, and yield of most crop plants are profound. Continuous exposure to drought conditions causes major alterations in the plant metabolism which ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is therefore a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance and/or tolerance to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold, and salt tolerance in model drought-, cold- and/or salt-tolerant plants are complex in nature- and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerant plants using biotechnological methods.
Therefore, what is needed is the identification of the genes and proteins involved in these multi-component processes leading to increased growth and/or increased stress tolerance. Elucidating the function of genes expressed in stress tolerant plants will not only advance our understanding of plant adaptation and tolerance to environmental stresses, but also may provide important information for designing new strategies for crop improvement.
Roots are an important organ of higher plants. Plant root systems are fundamental to the proper growth and development of all terrestrial plant species. In addition to uptake of water and nutrients and providing physical support, roots mediate a complex but poorly understood exchange of communication between soil microbes and other plants. In agronomic systems, production is impacted by the availability of water and nutrients in the soil: root growth has a direct or indirect influence on growth and yield of aerial organs, particularly under conditions of nutrient limitation. Roots are also relevant for the production of secondary plant products, such as defense compounds and plant hormones. Establishment of proper root architecture is an important factor for the plant to effectively use the water and nutrients available in the environment and to maximize plant growth and production. In addition, under conditions of drought, roots can adapt to continue growth while at the same time producing and sending early warning signals to shoots which inhibit plant growth above ground.
Moreover, improved root growth of crop plants will also enhance competitiveness with weedy plants and will improve growth in arid areas, by increasing water accessibility and uptake. Improved root growth is also relevant for ecological purposes, such as bioremediation and prevention/arrest of soil erosion. Longer roots can alleviate not only the effects of water depletion from soil but also improve plant anchorage and standability thus reducing lodging. Also, longer roots have the ability to cover a larger volume of soil and improve nutrient uptake. Therefore, altering root biomass, and in particular increasing root length, will improve plant growth as well as increase crop yield.
Roots are also storage organs in a number of important staple crops, for example, in sugar beets, potato, manioc (cassava), yams and sweet potato (batate). Roots are also the relevant organ for consumption in a number of vegetables (e.g. carrots, radish), herbs (e.g. ginger, kukuma) and medicinal plants (e.g. ginseng). In addition, some of the secondary plant products found in roots are of economic importance for the chemical and pharmaceutical industry, for instance, the basic molecules for the synthesis of steroid hormones is found in yams, and the roots of Lithospermum erythrorhizon produce shikonin, which is widely used because of its anti-inflammatory, anti-tumor and wound-healing properties.
Root architecture is an area that has remained largely unexplored through classical breeding because of difficulties with assessing this trait in the field. Thus, biotechnology could have significant impact on the improvement of this trait.
The structure of root systems results from a combination of genetic predisposition and physical environment. Additionally, soil microbes can also have a beneficial effect on plant growth either by reducing the deleterious effects of other pathogenic microbes, by producing compounds promoting plant growth, or by increasing the uptake efficiency of nutrients from the environment. A microbe may use one or all of these mechanisms at different stages of the plant life cycle.
The plant hormone ethylene has been implicated in a wide spectrum of plant growth and development processes. In plants, 1-aminocyclopropane-1-carboxylate (ACC) is an essential biosynthetic precursor of ethylene. Studies in the late 70's revealed that soil bacteria expressing an ACC deaminase can reduce the levels of ethylene in the plants thus leading to root elongation (Brown, 1974, Ann. Rev. Phytopathol. 12:181-197; Honma and Shimomura, 1978, Agric. Biol. Chem. 42:1825-1831). These results were supported by experiments where genes encoding ACC deaminase were mobilized into non-growth promoting bacteria which in turn, conferred a growth promoting effect (Shah et al., 1998, Can. J. Microbiol. 44:833-843). More recently, microbial genes encoding ACC deaminase were expressed directly in transgenic plants and shown to promote root elongation (Klee et al., 1991, Plant Cell 3(11):1187-93; Reed et al., 1996, J. Ag. Food Chem. 44(1):388-394).
Although some genes that are involved in stress responses in plants have been characterized, the characterization and cloning of plant genes that confer stress tolerance remains largely incomplete and fragmented. For example, certain studies have indicated that drought and salt stress in some plants may be due to additive gene effects, in contrast to other research that indicates specific genes are transcriptionally activated in vegetative tissue of plants under osmotic stress conditions. Although it is generally assumed that stress-induced proteins have a role in tolerance, direct evidence is still lacking, and the functions of many stress-responsive genes are unknown.
There is a need, therefore, to identify additional genes expressed in stress tolerant plants that have the capacity to confer increased root growth, and/or increased yield, and/or stress tolerance to its host plant and to other plant species. Newly generated stress tolerant plants will have many advantages, such as an increased range in which the crop plants can be cultivated by, for example, decreasing the water requirements of a plant species.